1. Field of Invention
The present invention relates to a multiport coupler for use in fiber optic data bus systems in which the optical signal attenuation between any two-port or remote terminal combination is substantially equal regardless of terminal location.
2. Description of Prior Art
Trends in aircraft avionic systems point to the use of fiber optic technology as a reliable means of digital data transmission of intra-aircraft signals between aircraft remote terminals. Data transmission concepts utilizing optical fibers offers several important advantages unmatched by conventional wire technology. A fiber optic link can carry extremely wideband information, is virtually immune to external electromagnetic disturbances, and will not interfere with existing aircraft systems. These properties make fiber optics an ideal transmission medium for applications where highly reliable data transmission is required. Of increasing importance, especially in commercial aircraft applications, is a data transmission system's immunity to lightning discharges. Metal aircraft skins provide some protection to signal wiring against the effects of lightning discharges, but the increased use of carbon fiber composite materials in aircraft to achieve weight performance advantages reduces this protection. Consequently, fiber optic data links offer an attractive way to provide such protection for intra-airplane signals.
Digital integration of advanced avionic systems offers potential weight savings, increased flexibility, and improved performance for future commercial and military aircraft. It is almost certain that these integration techniques will employ fiber optic digital data buses for efficient transfer of information between aircraft remote terminals.
The most popular hardwire serial data bus configuration utilizes tee couplers, or taps, dispersed along a transmission line. A remote terminal is then attached to each tee coupler. This configuration, while convenient, imposes severe limitations on fiber optic data buses. With current technology, series losses through many such tee couplers limits the data bus to a few remote terminals.
An alternate fiber optic data bus system that has been widely discussed is the star configuration. In this approach, a plurality of remote terminals connected to fiber optic cables are brought together at a single point by a star coupler. In this configuration, a signal from any one terminal is distributed equally to all other terminals. Performance of the star data bus is close to optimum, but the resulting cabling configuration is not convenient for aircraft applications where avionic equipment centers or remote terminals are widely separated or cable runs are severely restricted.
A third fiber optic data bus system designed specifically for aircraft application is the hybrid concept in which elements of the tee and star data buses are combined. In this approach, a plurality of remote terminals are joined at central points by power equalizing multiport optical couplers which are dispersed along a transmission line. As in the star configuration, the optical signal attenuation between all possible remote terminals is substantially the same regardless of terminal location. Such a configuration requires fewer connectors than does the tee configuration, has shorter cable runs than does the star configuration, and is very ammeanable to aircraft installation.
The key optical component in a fiber optic data bus configuration is the optical coupler. In such systems where signals must be divided between several remote terminals, it becomes important to insure that such division is uniform; otherwise some terminals receive a strong signal at the expense of others. The tee coupler, as exemplified in Brown, U.S. Pat. No. 3,902,786, is inherently lossy, consequently a data bus utilizing such couplers is restricted to applications where only a few remote terminals, normally less than 10, are required. The star coupler, of which Theil, U.S. Pat. No. 3,874,781, is an example, is more suited for data bus application; however, it imposes a severe constraint on the configuration of such a data bus and potentially may require long cable runs. On large commercial aircraft, such a configuration would be inconvenient to install.
A star coupler has the desirable property of providing a substantially equal attenuation between all two-port (or terminal) combinations in a star configured data bus. The power equalizing multiport optical coupler of the present invention achieves this result for the hybrid configured data bus. A hybrid configured data bus arbitrarily constructed with star couplers would not provide equal attenuations between all possible two-port combinations.